1. Field of the art
This invention relates to a process in which feed oils comprising alkylaromatic compounds are continuously hydrodealkylated by means of fluidized beds of particles which have hydrodealkylation activity.
2. Related art
Among the techniques heretofore known of catalytic hydrodealkylation of feed oils comprising alkylaromatic compounds, there are the processes which use chromiaalumina or cobalt-molybdenum oxide-containing particles or others as the catalyst.
Hydrodealkylation of alkylaromatic compounds is an exothermic reaction with more than about 10 kcal/mol for each methyl group when the alkyl is methyl. Generally, feed oils to be subjected to hydrodealkylation often contain polyalkylaromatic compounds and naphthenic or paraffinic hydrocarbons, and in such cases, hydrodealkylation of 2 to 5 mols of methyl groups per mol of feed oil, i.e., exothermic reaction of about 20 to 50 kcal/mol or more, occurs. In the prior art of hydrodealkylation, this large amount of heat generated by the reaction causes a wide temperature distribution and difficulty of temperature control in the reactor due to the nature of fixed bed reactor employed in the known process as described, for example, in Japanese Patent Publication No. 3024/64 and Japanese Patent Application Laid-Open Specification No. 13735/84. As a result, various problems tend to occur such as reduction of selectivity and generation of a large amount of tar or coke due to side reactions, deactivation of catalyst and plugging of the reactor.
In order to minimize these problems, such methods are tried or employed that, for example, the fixed bed reactor is made to have multi-stage structure and a large amount of cooling gas is introduced into the several parts in the reactor to reduce the temperature distribution, and/or feed oil is pre-treated and the reaction is conducted under a high hydrogen pressure higher than about 30 kg/cm.sup.2 to suppress the side reactions. However, any of these methods is not satisfactory to solve the problems because the effects are insufficient and the methods are not economical due to the sophisticated reactor structure and troublesome operation. Furthermore, operation of these processes must be frequently interrupted by decoking operation or catalyst replacement in a relatively short interval.
In U.S. Pat. Nos. 2,780,661 and 2,924,569, a process utilizing a fluidized bed as the reactor to reduce the temperature distribution in the reactor and to enable low temperature feed is described. However, these relate to a thermal hydrodealkylation process in which substantially inert particles such as sand or silica are used, but do not relate to the catalytic process described in this invention. Although those patents describe that the particles of 100 to 400 mesh in size should be used as fluidized particles, but to the best of our knowledge good fluidized state and homogenization of temperature throughout the reactor would not be attained with these particles. According to S. R. Beghea, R. L. Heinlich, A. B. Souby and L. T. Yule: Ind. Eng. Chem., 50, 1245 (1958) in which results of a pilot plant operation based on this method are described in detail, it is described that only poor fluidized state, i.e., severe slugging, was observed, and a large temperature distribution still remained in the reactor bed. Furthermore, this process is based on the same concept as that in the fixed bed processes that coke formation should be limited at as low a level as possible, and, therefore, a high pressure at 28 to 70 kg/cm.sup.2, preferably at about 42 kg/cm.sup.2 as well as a high concentration of hydrogen are necessary.
Such a high pressure, though it is effective to suppress or limit the coke formation, is much too high to be in the range in which the rate of hydrodealkylation as the main reaction is effectively promoted by the pressure, and brings about such inadvantage us aspects that it causes the unnecessary increase of the power consumption of compressors and the cost of reactor materials and, in the catalytic hydrodealkylation process, promotes undesirable hydrocracking of aromatic rings and lowers the selectivity of dealkylated products.
This invention proposes a new catalytic hydrodealkylation process in which high selectivity and activity can be maintained at relatively low hydrogen pressure, and the problems in the processes of the prior art mentioned above are solved.